Reactivation of latent herpesviruses is a particular problem in immunocompromised individuals, such as AIDS patients, who lack effective CD4 T helper cell function. The ability to mobilize residual immune defenses to combat opportunistic infections in the absence of CD4 T cells would represent a tremendous therapeutic advantage to these patients. Infection of mice with murine gammaherpesvirus-68 (MHV-68) provides a useful small animal model for studying the long-term control of persistent viral infection and for testing the ability of potential therapeutic agents to control viral reactivation. MHV-68 is a naturally-occurring rodent pathogen and is closely-related to the human pathogens Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus. MHV-68 replicates in the lungs of mice following intranasal administration and establishes a latent infection in B cells and epithelia. CD4 T cell-deficient mice can clear an initial challenge with virus, but fall to control latent virus, which reactivates in the lungs. Using this mouse model of opportunistic infection, we showed that treatment with an agonistic antibody to CD40 could substitute for CD4 T cell function and was highly effective in preventing reactivation of latent MHV-68 in lungs of CD4 T cell-deficient mice. Furthermore, our preliminary studies revealed that CD8 T cells are essential for this effect. The proposed studies are designed to dissect the mechanism by which anti-CD40 antibody treatment prevents viral reactivation in CD4 T cell-deficient mice, as follows: In Aim 1 we will determine the role of CD8 T cells. In Aim 2 we will determine the role of CD40-positive cells (such as B cells and dendritic cells). In Aim 3 we will determine the duration of the response, the number and frequency of treatments required and whether anti-CD40 treatment is effective against ongoing viral reactivation. The information obtained in these studies may be of significant value in designing novel immunotherapeutic agents, vaccines or protocols to combat viral reactivation in individuals with poor CD4 T cell function.